This invention relates to a composite substrate having a dielectric and an electrode, an electroluminescent (EL) device using the same, and a method for preparing the same.
The phenomenon that a material emits light upon application of an electric field is known as electroluminescence (EL). Devices utilizing this phenomenon are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices include dispersion type devices of the structure that a dispersion of a powder phosphor in an organic material or enamel is sandwiched between electrodes, and thin-film type devices in which a thin-film phosphor sandwiched between two electrodes and two insulating thin films is formed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film type EL devices have markedly spread the practical range of EL device application by virtue of a high luminance and a long lifetime.
In prior art thin-film type EL devices, the predominant structure is such that blue sheet glass customarily used in liquid crystal displays and plasma display panels (PDP) is employed as the substrate, a transparent electrode of ITO or the like is used as the electrode in contact with the substrate, and the phosphor emits light which exits from the substrate side. Among phosphor materials, Mn-doped ZnS which emits yellowish orange light has been often used from the standpoints of ease of deposition and light emitting characteristics. The use of phosphor materials which emit light in the primaries of red, green and blue is essential to manufacture color displays. Engineers continued research on candidate phosphor materials such as Ce-doped SrS and Tm-doped ZnS for blue light emission, Sm-doped ZnS and Eu-doped CaS for red light emission, and Tb-doped ZnS and Ce-doped CaS for green light emission. However, problems of emission luminance, luminous efficiency and color purity remain outstanding until now, and none of these materials have reached the practical level.
High-temperature film deposition and high-temperature heat treatment following deposition are known to be promising as means for solving these problems. When such a process is employed, use of blue sheet glass as the substrate is unacceptable from the standpoint of heat resistance. Quartz substrates having heat resistance are under consideration, but they are not adequate in such applications requiring a large surface area as in displays because the quartz substrates are very expensive.
It was recently reported that a device was developed using an electrically insulating ceramic substrate as the substrate and a thick-film dielectric instead of a thin-film film insulator under the phosphor, as disclosed in JP-A 7-50197 and JP-B 7-44072.
FIG. 2 illustrates the basic structure of this device. The EL device in FIG. 2 is structured such that a lower electrode 12, a thick-film dielectric layer 13, a light emitting layer 14, a thin-film insulating layer 15 and an upper electrode 16 are successively formed on a substrate 11 of ceramic or similar material. Since the light emitted by the phosphor exits from the upper side of the EL structure opposite to the substrate as opposed to the prior art structure, the upper electrode is a transparent electrode.
In this device, the thick-film dielectric has a thickness of several tens of microns which is about several hundred to several thousand times the thickness of the thin-film insulator. This offers advantages including a minimized chance of breakdown caused by pinholes or the like, high reliability, and high manufacturing yields.
Use of the thick dielectric invites a drop of the voltage applied to the phosphor layer, which is overcome by using a high-permittivity material as the dielectric layer. Use of the ceramic substrate and the thick-film dielectric permits a higher temperature for heat treatment. As a result, it becomes possible to deposit a light emitting material having good luminescent characteristics, which was impossible in the prior art because of the presence of crystal defects.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is about one hundredth of the thickness of the thick-film dielectric layer. This requires the surface of the thick-film dielectric layer to be smooth at a level below the thickness of the light emitting layer. However, a conventional thick-film technique was difficult to form a dielectric layer having a fully flat and smooth surface.
If the surface of the dielectric layer is not flat or smooth, there is a risk that a light emitting layer cannot be evenly formed thereon or a delamination phenomenon can occur between the light emitting layer and the dielectric layer, substantially detracting from display quality. Therefore, the prior art method needed the steps of removing large asperities as by polishing and removing small asperities by a sol-gel process.
However, it is technically difficult to polish large surface area composite substrates for display and other applications. The sol-gel process cannot accommodate for large asperities when used alone. Additionally, an increased cost of stock material and an increased number of steps involved are undesirable.
An object of the invention is to provide a method for preparing a composite substrate, which prevents an insulating layer surface from becoming rugged under the influence of an electrode layer, and eliminates a polishing step, whereby the composite substrate is easy to manufacture and ensures high display quality when applied to thin-film light emitting devices, the composite substrate and an EL device using the same.
The above object is attained by the following construction.
(1) A method for preparing a composite substrate, comprising the steps of:
successively applying an electrode paste and an insulator paste onto an electrically insulating substrate as thick films to form a composite substrate precursor having a green electrode layer and a green insulator layer laminated,
subjecting the precursor to pressing treatment using a die press or roll, for smoothing its surface, and
firing to complete the composite substrate.
(2) The method for preparing a composite substrate according to (1), wherein during the pressing treatment, the die or roll used for pressing is held at a temperature in the range of 50 to 200xc2x0 C.
(3) The method for preparing a composite substrate according to (1) or (2), wherein the electrode paste and/or the insulator paste uses a thermoplastic resin as a binder.
(4) The method for preparing a composite substrate according to any one of (1) to (3), wherein during the pressing treatment, a resin film having a parting agent thereon is interposed between the die or roll and the green dielectric layer.
(5) A composite substrate obtained by the method of any one of (1) to (4), wherein a functional thin film is to be formed on the thick-film dielectric layer.
(6) An EL device comprising at least a light emitting layer and a transparent electrode on the composite substrate of (5).
(7) The EL device of (6) further comprising a thin-film insulating layer between the light emitting layer and the transparent electrode.
According to the invention, a composite substrate of substrate/electrode/insulator layer having a thick-film insulator layer with a smooth surface can be prepared by a simple step of carrying out compression on an unfired dielectric layer.
When an EL device is prepared using the composite substrate having an insulator layer with a smooth surface, a light emitting layer to lie thereon can be formed uniformly without giving rise to a delamination phenomenon. As a result, an EL device having improved light-emitting performance and reliability can be fabricated. The compression step is compliant with large surface area displays because of an eliminated need for polishing step which was necessary in the prior art, and reduces the manufacturing cost because of a reduced number of steps.